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Listener Callback Syntax

Specifying Listener Callbacks

Callbacks are functions that execute when the listener receives notification of the event. Pass a function handle referencing the callback function to handle.addlistener or handle.listener when creating the listener.

All callback functions must accept at least two arguments:

  • The handle of the object that is the source of the event

  • An event.EventData object or an object that is derived from the event.EventData class.

Syntax to Reference Callback

For a function: functionName

lh = addlistener(eventSourceObj,'EventName',@functionName)

For an ordinary method called with an object of the class: obj.methodName

lh = addlistener(eventSourceObj,'EventName',@obj.methodName)

For a static method:ClassName.methodName

lh = addlistener(eventSourceObj,'EventName',@ClassName.methodName)

For a function in a package:PackageName.functionName

lh = addlistener(eventSourceObj,'EventName',@PackageName.functionName)

Input Arguments for Callback Function

Define the callback function to accept the required arguments:

function callbackFunction(src,evnt)
   ...
end

If you do not use the event source and event data arguments, you can define the function to ignore these inputs:

function callbackFunction(~,~)
   ...
end

For a method:

function callbackMethod(obj,src,evnt)
   ...
end

Additional Arguments for Callback Function

To pass arguments to your callback in addition to the source and event data arguments passed by MATLAB®, use an anonymous function. Anonymous functions can use any variables that are available in the current workspace.

Syntax Using Anonymous Function

Here is the syntax for an ordinary method. The input arguments (arg1,...argn) must be defined in the context in which you call addlistener.

lh = addlistener(src,'EventName',@(src,evnt)obj.callbackMethod(src,evnt,arg1,...argn)

Use varargin to define the callback function.

function callbackMethod(src,evnt,varargin)
   arg1 = varargin{1};
   ...
   argn = varargin{n};
   ...
end

For general information on anonymous function, see Anonymous Functions.

Using Methods for Callbacks

The TestAnonyFcn class shows the use of an anonymous function with an additional argument. The listener callback displays the inputs arguments to show how MATLAB calls the callback method.

classdef TestAnonyFcn < handle
   events
      Update
   end
   methods
      function obj = TestAnonyFcn
         t = datestr(now);
         addlistener(obj,'Update',@(src,evnt)obj.evntCb(src,evnt,t));
      end
      function triggerEvnt(obj)
         notify(obj,'Update')
      end
   end
   methods (Access = private)
      function evntCb(~,~,evnt,varargin)
         disp(['Number of inputs: ',num2str(nargin)])
         disp(evnt.EventName)
         disp(varargin{:})
      end
   end
end

Create an object and trigger the event by calling the triggerEvt method:

obj = TestAnonyFcn;
obj.triggerEvnt;
Number of inputs: 4
Update
01-Jul-2008 17:19:36

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